A fleet of miniature helicopter drones mounted with thermal imaging cameras will be deployed to combat graffiti-spraying gangs on the German railway network.

The drones, which fly at an altitude of 150 yards, will be used at graffiti 'hotspots' such as the big German cities of Berlin, Leipzig, Cologne and Hamburg, a spokesman for Deutsche Bahn confirmed.

The use of drones against vandals is the latest indication of the growing civilian market for unmanned aerial reconnaissance. Over 400 new drone systems are being developed by firms based in Europe, according to an EU report published last September.

The drones used by Deutsche Bahn cost 60,000 euros each and are manufactured by German firm Microdrones, which also markets the machines for landscape photography, analysing traffic accidents and monitoring crops.

The drones are a metre wide, from the tip of one rotor arm to another. They will be painted in Deutsche Bahn's red livery.

Jens-Oliver Voss, a spokesman for Deutsche Bahn, said that because of Germany's privacy laws the drones would not be used near stations, or other areas where members of the public might be caught on film.

"The drones will not be used at stations, but at places like depots. They can be used to search large areas, which are difficult to patrol on foot," Mr Voss said.

"They will be used in combination with security guards, identifying graffiti-sprayers so they can be apprehended by security forces, who can hand them over to police." Deutsche Bahn is refusing to say how many of the drones it has purchased.

Removing graffiti costs Deutsche Bahn 7.6million euros a year.

The machines, which can fly for up to 80 minutes at a speed of 33 mph, can operate autonomously or be remotely controlled by a human operator. The drones' motors emit little noise, making them ideal for surveillance.

Three years ago, police in Merseyside made their first arrest using a drone, which tracked a suspected car thief who attempted to hide in undergrowth.